In a typical aircraft fuel system consisting of a positive-displacement pump and bypassing fuel metering unit, a portion of the pump flow is supplied to the combustor to be burned by the engine while another portion of the flow is supplied as accessory flow. For the purposes of this application, accessory flow includes actuation flows, flows used for cooling, servos, and leakage. Typically, the accessory return flow and any excess pump flow are returned to the low pressure inlet of the pump. The positive-displacement aircraft pump is typically sized by either the pump fuel flow required for a relight condition, or for the maximum pump fuel flow required at takeoff condition. The total pump flow required at these conditions includes engine burn flow plus the accessory flows.
The main fuel pumps for turbine-engine aircraft are typically high-pressure, positive-displacement pumps. Generally, the pump flow rates in these pumps is proportional to engine speed. At many engine operating conditions, the engine flow demand is significantly less than the high pressure flow supplied by the main pump. The excess high-pressure pump fuel flow is typically bypassed back to the low pressure inlet of the pump. However, the energy required to raise the pressure of this excess fuel flow, and to return the excess flow back to the low pressure inlet, is effectively wasted. This energy is realized as heat input to the fuel, and results in undesirable higher fuel temperatures under certain conditions.
It would therefore be desirable to have a fuel distribution system and a control system therefor capable of reducing this excess fuel flow, and the wasted energy associated with the operation of these systems. Embodiments of the invention provide such a fuel distribution and control system. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.